1. Field of the Invention
The present invention relates to an image processing apparatus that, when a document size is indefinite, perform a discrimination processing on an image area or an image type in the document, and an image forming apparatus, an image processing method and a recording medium on which the image processing program is recorded.
2. Description of the Related Art
In an image forming apparatus such as a copier and multi-function peripheral, and in an image reading apparatus such as a scanner, when a so-called determination of document features, such as a document type determination, a color/monochrome determination and a page background determination, is performed, a threshold used for the determination is normalized by a document size of an input image, and a determination process is performed by using the normalized threshold.
For example, in an image processing apparatus described in Japanese Unexamined Patent Publication JP-A 8-186726 (1996), the document size is detected and, in an AE process, a histogram of monochrome pixels is generated based on the detected document size. A page background level is detected by analyzing the generated histogram.
In an image forming apparatus described in Japanese Unexamined Patent Publication JP-A 9-46512 (1997), the document size is detected, a density histogram of an input image data is generated and a black run length of the input image data is calculated. Based on the generated histogram and the calculated black run length, the discrimination of the document type and the detection of the page background level are performed.
In the case of setting a threshold to be used for the determination process based on the document size, it is necessary to determine the document size by placing a device for detecting the document size or for a user to manually input the document size.
For example, in apparatuses without the device for detecting the document size, the user needs to recognize the document size and perform input operation, but the user needs to have a knowledge of the document size and perform the input operation.
On the other hand, when the user is not prompted to input the document size, the discrimination process is performed by setting an object area in which the discrimination of the document features is performed.
FIG. 12 is a view showing an example of the discrimination object area which has been set. When a smaller area compared to a maximum document size readable by the image forming apparatus is set as a discrimination object area 101, since the document features are discriminated based on a part of the area of the document, it is possible to determine the page background with relatively high accuracy, but the accuracy of color/monochrome determination and the document type determination decreases.
FIG. 13 is a view showing an example of the discrimination object area which has been set. When reading is performed including an area outside of the document size and the area read is set as a determination object area 102, since information of an entire document can be read, the document type determination and the color/monochrome determination are possible. However, the page background determination lacks accuracy since a pressing plate of a document cover is included within the discrimination object area and information of this pressing plate is inputted. To deal with this problem, it is possible to color the pressing plate black or the like, but in this case the image data which has been read may suffer offset.